1. Field of the Invention
The present invention relates generally to an automatic power transmission for an automative power train. More specifically, the invention relates to a lubricant circulating arrangement for an automatic power transmission for circulating lubricating fluid for lubricating components of the automatic power transmission.
2. Description of the Background Art
In general, an automatic power transmission includes a lubricating circuit for circulating pressurized lubricating fluid therethrough for lubricating necessary components. For this purpose, lubricating fluid flowing back via a pressure relief valve or from an oil cooler, is recirculated, as illustrated in Japanese Patent First Publication No. 62-62047, for example. Separately from the above, pressurized lubricant fluid from a pressurized fluid source can be directly used for lubrication. In the latter case, in order to prevent an excess amount of lubricating fluid from flowing through a lubrication circuit, a flow restriction device is provided therein. As a flow restriction device, a choke is generally used for a high flow restriction capacity.
However, when the choke is used in the lubricating circuit, an excess amount of lubricating fluid can flow into the lubricating circuit. Particularly, when the lubricating fluid is at a high temperature and thus has a low viscosity, such tendency is increased. As a result, lack of lubricating fluid pressure or flow rate can be caused in the pressurized fluid source. On the other hand, when excess amount of lubricating fluid is supplied in the lubricating circuit, frictional resistance due to viscosity of the lubricating fluid is increased. Reduction of a possibility of excess flow of the pressurized lubricating fluid can be obtained by utilizing smaller path area choke. However, reduction of the path area of the choke is practically difficult because of difficulty in machining. Furthermore, even when a smaller path area choke is technically possible to manufacture, such smaller path area may increase a possibility of blocking due to foreign matters, such as dust, dirt or so forth, contained in the lubricating fluid.